Many analog circuits, such as integrators, active filters, and the like, use differential signalling. Unlike single-ended signalling in which a voltage varies with respect to a fixed voltage, such as a ground voltage, differential signalling conveys a signal as a difference between two voltages. A differential signal includes two components, a differential component and a common-mode component. While the differential component conveys the analog signal, the common-mode component is also important. The common-mode voltage of a differential signal determines the available headroom for subsequent amplifiers and filters. In general, it is desirable to have the common-mode voltage set at the midpoint between positive and negative power supply voltages. If the common-mode voltage varies too much from the supply midpoint, operating headroom is reduced, which may cause distortion when a subsequent amplifier processes the differential signal.
Switched-capacitors are generally equivalent to resistors, and are more easily manufactured in integrated circuit form. Thus, switched-capacitor circuits are often useful in differential signal processing circuits. A switched-capacitor circuit used as an input sampler samples an input signal during a sample phase and transfers the input signal to the input terminals of a fully-differential amplifier during a transfer phase. A fully-differential switched-capacitor input sampler may be adapted to sample a single-ended signal by connecting one of the inputs to an analog ground terminal. However, known switched-capacitor input samplers have no mechanism to guarantee the equality of charge on corresponding capacitors in the input sampler. If equality is not maintained, then one of the amplifier's outputs may eventually become displaced more than the other relative to the common mode, which tends to shift the output common-mode voltage.
Another use of switched-capacitor circuits is in common-mode feedback circuits. One way to sense the common-mode voltage between a differential signal is to use resistor averaging, as taught by Paul M. Henry in U.S. Pat. No. 4,105,942. Two equal-valued resistors are connected in series between an amplifier's positive and negative output terminals. The interconnection point between the resistors averages the two voltages, and thus represents the sensed common-mode voltage. This voltage is fed back to the amplifier, which adjusts the common-mode output voltage until it equals a reference voltage. However, the use of resistors is generally not desirable considering available integrated circuit technology. In order to have resistance values large enough to have low current drain, excessive integrated circuit area would be required.
It is possible to sense the common-mode voltage using two capacitors connected in series between the differential signal lines. However, the use of capacitors to sense the common-mode voltage creates an additional problem: any initial offset will remain on the capacitors, offsetting the common-mode voltage from the reference voltage. Including high-valued resistors in parallel with the capacitors to discharge the initial offset is not very helpful either, because these resistors would require a large amount of integrated circuit area.
One alternative known in the prior art is to substitute switched-capacitors for resistors to periodically discharge the continuous-time sampler. While avoiding the need for a large circuit area, however, switched-capacitors still consume significant amounts of current when they repeatedly discharge the fixed capacitors. In addition, switched-capacitors cause large instantaneous current transients on the output signal lines, which may result in undesirable harmonic distortion on the output signal if the amplifier is not strong enough to avoid going into a current-limit condition. What is needed, then, is a switched-capacitor circuit which overcomes the problems with switched-capacitor networks when used as input samplers, common-mode feedback circuits, and the like.